1. Field of the Invention
The present invention relates to an electroluminescence (EL) display device and a method of manufacturing the same, and more particularly, to an EL display device with improved external light coupling efficiency and brightness that can be easily manufactured and a method of manufacturing the same.
2. Description of the Related Technology
The external light coupling efficiency ηex of an EL display device is given by the following equation:ηex=ηin·ηout  Equation (1)where, ηin and ηout respectively denotes the internal quantum efficiency and the output quantum efficiency of the EL display device. The internal quantum efficiency ηin is determined by self-elimination within each layer, and the output quantum efficiency ηout is determined by the prevention of light from being emitted to the outside due to total internal reflection in each layer (i.e., failure to transmit the light to the outside due to total internal reflection occurring at an interface because the incident angle is greater than a critical angle when the light is incident from a medium having a higher refractive index to another medium having a lower refractive index). In the EL display device, the light emitted from a light emitting layer transmits through many layers before being emitted to the outside, and thus there is light which is not emitted to the outside due to the variations in the refractive indexes of the layers.
The output quantum efficiency ηout, that is, the light transmittance efficiency, which depends on the total internal reflection at interfaces between the layers, when the light output from the luminescence layer is emitted to the outside can be expressed as:
                              1          2                ⁢                              (                                          N                out                                            N                                  i                  ⁢                                                                          ⁢                  n                                                      )                    2                                    Equation        ⁢                                  ⁢                  (          2          )                    where Nin is the refractive index of the layer from which light is being emitted, and Nout is the refractive index of the layer into which the light is entering. For example, the output coupling efficiency of light transmitting through a layer with a refractive index of about 1.5 to a layer with a refractive index of about 1.2 is determined to be 32% using Equation 2. That is, about 70% of the light which enters the interface is not emitted to the outside.
There has been many attempts to increase external light coupling efficiency.
Japanese Patent Publication No. 4-192290 discloses an inorganic EL device wherein a plurality of condensing microlenses being the same size or larger than an inorganic EL element are formed on an external surface of a transparent substrate on which the inorganic EL element is formed. Light incident on the interface between the transparent substrate and air at an angle larger than the critical angle nonetheless has an incident angle less than the critical angle on the microlenses, thereby is not totally internally reflected. Also, the light is emitted in a predetermined direction to improve brightness. However, the EL element is a surface light source in the cited invention, and thus diffused light that is not focused inevitably occurs when using a microlens that is the same size or larger than the EL element. In addition, clarity of an image is reduced due to overlapping images produced by adjacent EL elements.
Japanese Patent Publication No. 7-037688 discloses an EL element formed on a substrate having a cylindrically-shaped high refraction factor section composed of a material having a higher refraction index than the surrounding material formed around the side surface of the substrate. Light produced by the EL element is emitted through the high-refraction factor section to increase external light coupling efficiency. However, in the cited invention, the light which transmits through the high-refraction factor section is diffused light, as illustrated in FIG. 1 of the cited invention. Thus, the brightness of the light emitted to the front is not improved that much.
Japanese Patent Publication No. 10-172756 discloses an organic EL light emitting device in which one or a plurality of condensing lenses are formed between a lower electrode composing the organic EL light emitting device and a transparent substrate. The organic EL light emitting device and the condensing lens are disposed to correspond to each other. Light transmitted through the EL light emitting device is incident on an interface of the transparent substrate at a smaller angle than the critical angle to increase the external light coupling efficiency. However, in the cited invention, overlapping images produced by adjacent EL light emitting devices reduce image clarity.